As a licensed builder I can say the best idea is going to a 4x4 site and asking advice about your house.

The amount of variables that need to be answered before you can make a decision on this prove this an impossible question to answer without a visit to the house to diagnose the conditions. And no, a crawl space does not necessarily need to be conditioned. Do you want to add that much area to your heat bill?

Call someone in to look at this before you just throw money at it.

1) It's the pub

2) Just because it's a 4x4 site doesn't mean there aren't intelligent people with good info. Sure, it's a 4x4 site....do you think I'm going to do exactly as someone on here says word for word? It's the internet...there's wrong answers everywhere (including building forums by hack builders).

3) You mention heating bill going up with a conditioned space. Everything I've read (scientific reports by unbiased non government agnecies) says that while your heating bill may go up in the coldest months of winter, overall it typically goes down....thoughts?

4) I get that it doesn't NEED to be conditioned. It hasn't been conditioned for more than two decades. The point is that the damn insulation eventually falls from the joists, doesn't provide much insulation factor, and I want to fix that...

Back on topic...I found this place here, and they're in Wixom, so I plan to discuss some things with them. Biased opinion, but they're right down the road.

Regardless what I do with the ventalation, everything I read from anyone says insulating your foundation results in much higher energy savings. Pretty disputable there.